This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We have previously shown that tendon and demineralized bone re-calcify rapidly when incubated at 37[unreadable]C in rat serum (1). The objective of the present study is to examine the ultrastructural location of the initial mineral deposited into these type 1 collagen matrices during incubation in serum. Our working hypothesis is that the initial mineral will be deposited within the hole region of the collagen fibril, as has been seen in the initial stages of mineralization in vivo.